The present invention generally relates to a capacitive vacuum measuring cell.
It is known that pressures or pressure differences can be measured by applying pressure to a thin membrane and measuring its deflection. A known and suitable method for measuring the deflection is to design the membrane arrangement as a variable electrical capacitance where the capacitance change which correlates with the pressure change is evaluated by measurement electronics in the known manner. The capacitance is created by arranging a thin, flexible membrane very close to another surface and by depositing an electrically conductive film on both mutually opposed surfaces or by fabricating them from electrically conductive material. When pressure is applied to the membrane, the deflection changes the distance between the two electrodes which leads to an analyzable capacitance change of the arrangement. Sensors of this type are mass-produced from silicon. The flat basic body, as well as the membrane, often consist entirely of silicon. There are also versions that are made of composite materials such as silicon with a glass substrate. Such sensors can be produced very economically. However, in vacuum applications, pressure sensors of this type are normally usable only for higher pressures in the range of approx. 10xe2x88x921 mbar to several bar. High resolution at pressures below 10xe2x88x921 mbar is no longer achievable with silicon. One of the reasons for this is that the silicon surface reacts with the environment, which impairs the sensitive sensor characteristic. Already water vapor that forms part of normal atmospheric air leads to a corresponding reaction on the surfaces. The problem becomes even more serious when the sensor is used in chemically aggressive atmospheres. For this reason, attempts were made to protect such silicon sensors against external influences by passivating the surfaces. Attempts were also made to deposit protective coatings on the surfaces in order to improve the durability and the resistance against chemically aggressive environments as described in DE 41 36 987. Such measures are costly and, in the case of mechanically deformable parts such as membranes, have only limited success, in particular in highly aggressive media such as fluorine, bromic acid and their compounds which are typically used in vacuum etching processes.
For this reason, attempts were made to build vacuum measuring cells entirely from corrosion resistant materials such as Al2O3. A known arrangement of this type is shown in FIG. 1. The vacuum measuring cell consists of a ceramic plate (20) above which a membrane (22) is arranged with a small gap between the two of them and a fusible seal (21) between the ceramic plate (20) and the edge of the membrane. The ceramic plate (20) together with the membrane (22) forms a reference vacuum chamber (25) that is evacuated down during the manufacturing process through a pumping port and which is sealed with a seal (28). The mutually opposed surfaces of the ceramic plate (20) and the membrane (22) inside the reference vacuum chamber (25) are coated with electrically conductive material and connected to insulated external terminals on which the capacitance signal can be evaluated by means of an electronic device (not shown in the illustration). To achieve corrosion resistance, plate (20) and membrane (22) are both made of ceramic material such as Al2O3. This vacuum measuring cell in turn is arranged in a vacuum-tight housing (23) that features a port (24) which is connected to the media to be measured. Via port (24) of the vacuum measuring cell, the resulting measurement vacuum chamber (26) is sealed off against the membrane (22) by means of an elastomer seal (27) so that the pressures to be measured reach only the surface of the membrane (22). For the purpose of sealing, the entire cell is pressed via the ceramic plate (20) and membrane (22) against the elastomer seal (27). Up to now, vacuum measuring cells of this type have been usable only for higher pressures in the range of 0.1 mbar to 100 bar. In addition, this design leads to stress in the materials which, at lower pressures, for example  less than 1 mbar, significantly impairs the reproducibility of measurement results and the resolution. The ceramic membranes (22) used so far have a thickness ranging from 279 xcexcm to 2540 xcexcm. Such designs are not suitable for achieving wide measurement ranges, in particular low pressures of 0.1 mbar to 10xe2x88x926 mbar. In addition, designs of this type, as disclosed also in U.S. Pat. No. 5,553,502, are costly.
The objective of the present invention is to eliminate the disadvantage of the current state of the art. In particular the objective of the present invention is to implement an easy-to-produce, economical vacuum measuring cell that is suitable for measuring pressures from 10xe2x88x926 mbar to 1000 mbar, in particular from 10xe2x88x926 mbar to 1 mbar, with an accuracy of better than 1%, preferably better than 0.3% of the measured value. The measurement range can be covered or subdivided into several vacuum measuring cells or membrane versions according to the invention. In addition this vacuum measuring cell shall be corrosion resistant to aggressive media, have a compact design, and be economical to manufacture.
The capacitive vacuum measuring cell according to the invention is made entirely out of ceramic, in particular Al2O3. This results in high corrosion resistance and long term reproducibility. Only in the areas where sealing is required or where feedthroughs are provided are small amounts of materials other than Al2O3 used, if the Al2O3 is not fused without addition of the foreign material. A vacuum measuring cell consists of a first plate-shaped housing body above which a membrane, sealed along its edges, is arranged so that it encloses a reference vacuum chamber. On the side pointing away from the reference vacuum chamber, there is a second housing body, also sealed along its edges, so that a measurement vacuum chamber is formed there. This measurement vacuum chamber features a port for connecting the medium to be measured. The surface of the first housing body and the membrane that form the reference vacuum chamber are coated with an electrically conductive film, for example, gold, and constitute the electrodes of the capacitance measuring cell. The electrodes are led out, for example, through the first housing body or through the sealing area in the edge zones. The essentially parallel electrode surfaces are spaced apart from 2 xcexcm to 50 xcexcm. Sealing of the membrane in the edge zone against the two housings is preferably achieved through welding, for example, laser welding. Highly suitable, and simple to use, is also a glass soldering and/or brazing material that is corrosion resistant. Another possibility of achieving a sealing bond is to connect the housing parts diffusively, for example, in the green body state in which the objective is to completely avoid material other than Al2O3.
The measuring cell arrangement according to the invention essentially allows a symmetric design that avoids all stress in the housing. This is particularly important in order to achieve high measurement sensitivity combined with high accuracy and reproducibility. It also allows the utilization of a very thin ceramic membrane which is essential for reliably measuring vacuum pressures lower than 100 mbar, and in particular lower than 10 mbar, by means of capacitive, all-ceramic measuring cells. For this purpose, membrane thicknesses of 10 xcexcm to 250 xcexcm are needed, where membrane thicknesses of 10 xcexcm to 120 xcexcmare preferred in order to achieve a very good resolution. Typical membrane thicknesses are, for example:
Such thin membranes are very difficult to manufacture and, after the sinter step, require at least one additional smoothing step. It is also very important for the membrane to be sufficiently helium tight which can be achieved only if the grain size of the membrane material is not too large and remains within the range of  less than 20 xcexcm. Smaller grain sizes of  less than 10 xcexcm are preferred, in particular, those  less than 5 xcexcm. In all cases, the cross-section of the membrane viewed across the thickness should contain at least two grains; the membranes are particularly tight if more than five grains are on top of each other.
Another important criterion for achieving an accurate measuring cell is the planarity of the membrane surface. The unevenness across the entire surface should in all cases not exceed 30% of the electrode gap, preferably not more than 15%. This means that the unevenness across the entire surface should not exceed 10 xcexcm, preferably no more than 5 xcexcm. The unevenness is defined here as the difference between the lowest and the highest point. To achieve the desired long-time stability, the purity of the aluminum oxide used for the membrane should be at least 94%, with preferred values being above 99%.
To ensure that the quality of the membrane seal in the edge zone is not impaired, it is advantageous to lead out the electrically conductive layers via feedthroughs that are arranged on the first housing body, rather than directly through the membrane seal or fused joint.
To ensure accurate functioning of the measuring cell over a long period of time, the reference vacuum chamber must have a high-quality vacuum with long-time stability. After evacuation, a getter should be provided that is preferably arranged in a small volume on the first housing and communicates with the reference vacuum chamber. This getter ensures that the reference vacuum pressure is lower, preferably by at least one decade, than the pressure to be measured. To prevent contaminations of the internal measuring cell space, a getter type that is not evaporating should be chosen.
Measuring cells designed according to the invention can be very compact and economical to produce. The diameter of such a cell can be between 5 and 80 mm where the measuring cell preferably has a diameter of 5 to 40 mm. The thickness of such a cell is preferably in the range of 2 mm to 25 mm.